This invention relates to backlit liquid crystal display devices (LCDs) having a first light source for operation during the day, and a second light source for operation at night.
LCDs (i.e. liquid crystal display devices) are gaining in popularity for use in systems such as television receivers, computer monitors, avionic displays, aerospace displays, and other military-related displays where the elimination of cathode ray tube (CRT) technology is desirable for several reasons. In particular, cathode ray tubes are characterized by large depth dimensions, undesirably high weight, and fragility. Additionally, cathode ray tubes (CRTs) require a relatively high voltage power supply in order to sufficiently accelerate electron beams for displaying images.
The aforementioned shortcomings of cathode ray tubes are overcome by flat panel liquid crystal displays in which matrix arrays of liquid crystal picture elements or pixels are arranged in a plurality of rows and columns. Patterns of information are thereby defined by the two dimensional array of pixels, which because of differences in the orientation of the liquid crystal material within each pixel, are caused to appear either darkened or transparent.
Liquid crystal displays may be either transflective or transmissive. Transflective displays depend upon ambient light conditions in order to be viewed, i.e. light from the surrounding environment, wherein ambient light incident upon the side of the display facing the viewer is reflected back to the viewer and allows the display to be viewed by observers. Transflective liquid crystal displays cannot, therefore, be used in dark or low light environments, because there is no light available for reflection back to the viewing surface of the display.
Conversely, transmissive or backlit liquid crystal displays require the use of illuminating means, such as a tubular or serpentine fluorescent lamp array, operatively disposed on the side of the matrix array opposite the viewer. This illumination means, or backlight, may also include a back reflector adapted to efficiently redirect any stray illumination towards the matrix array of pixels, thus insuring that the displayed image is as bright as possible (given the characteristics of the lighting scheme employed).
In the past, a large amount of research in the field of flat panel liquid crystal devices has been dedicated to the design of backlighting schemes which optimize viewing and structural parameters of liquid crystal displays. Particular focus has been on the desirablity of achieving substantial uniformity and high intensity of light across the illuminated matrix area while maintaining low power consumption and a low overall profile, i.e. a thin assembly.
For example, as disclosed in commonly assigned U.S. Pat. No. 5,161,041, the entire disclosure of which is incorporated herein by reference, a lighting assembly for a backlit color LCD includes an integrally formed image splitting/collimating lens for effectively enlarging the area illuminated by any one or part of one of the backlighting lamps. This integral image splitting and collimating lens has the advantages of providing a bright, uniform light to the matrix array of pixels while maintaining a narrow profile and minimizing the power consumption of the display. This bright, uniform lighting scheme achieves a high contrast display in bright ambient light conditions. The effect of the integral image splitting and collimating lens is to eliminate local bright and pale spots in the display corresponding to the legs and spaces between the legs of a typical serpentine fluorescent lamp, by providing two similar images of the light emanating from each lamp leg. By locating the split images substantially contiguous, one to the other, the area of illumination is effectively enlarged and a bright, uniform light distribution across a low profile LCD is obtained. Additionally, when a light diffuser is provided between the integral lens and the matrix array, wide angle viewability is achieved. The precise diffuser chosen depends on the specific application of the liquid crystal display.
In preferred embodiments of U.S. Pat. No. 5,161,041, the integral image splitting and collimating lens includes a thin film having light refracting, faceted prisms formed on one of its faces. An example of such a film is 3M SCOTCH.TM. Optical Lighting Film, described in "3M Scotch.TM. Optical Lighting Film General Theory" (November, 1988), the disclosure of which is incorporated herein by reference. In preferred forms, this thin 3M Scotch film is laminated onto a clear transparent substrate of, for example, glass, ceramic or plastic (most preferably glass).
While the liquid crystal displays of U.S. Pat. No. 5,161,041 have a low profile, improved lighting efficiencies, and excellent optical properties, only one light source or lamp is used for both day and nighttime operation. In many cases, particularly military uses, it becomes necessary to be capable of viewing a liquid crystal display both during the day, and during dark conditions when the viewer is utilizing night vision electronic equipment, such as night vision goggles (NVG). Such night vision electronic equipment is designed to be sensitive to very low light level intensities, frequently in the near infrared region. Any near infrared source of light at an intensity above the surrounding nighttime conditions will have the tendency to cause night vision electronic viewing equipment (e.g. NVG) to overload and cease functioning. Quite clearly then, LCDs designed for daylight and nighttime use which employ a single lighting source that emits IR energy becomes a handicap when night vision goggles (NVG) etc. are to be employed.
One solution to the above-described problem is to dim the light source of aforesaid mentioned U.S. Pat. No. 5,161,041 to sufficiently low intensity levels required for nighttime use. The backlit liquid crystal display light source(s) of U.S. Pat. No. 5,161,041, when dimmed to the low intensity levels required for nighttime use, produce excellent low intensity light rays. However, when dimmed to very low intensity levels, many fluorescent lamps tend to lose their stability and uniformity. Loss of stability is defined herein to mean that the fluorescent lamps may begin to flicker. Loss of uniformity is defined herein to mean that dark and light bands or strips appear along the fluorescent lamps.
Night vision equipment operates, as aforesaid, because of high sensitivity to very low levels of light, mainly in the near infrared region of the spectrum (i.e. about 630-1,100 nm). Efforts to block the infrared (IR) emissions of liquid crystal displays and other display panel equipment have largely been unsuccessful because color integrity (particularly of the color red) and the ability to view LCDs at wide viewing angles from normal (e.g. up to about 30.degree.-60.degree. from normal) could not be achieved. This is particularly true when it comes to achieving these results in the highly advantageous active matrix liquid crystal displays.
The problems associated with achieving acceptable IR blockage, while maintaining color integrity and wide viewing angles, are reported and demonstrated in Abileah et al., "A Full Color AML With NVG Class B Compatibility" IEEE, AES Magazine (March, 1992), pp. 1237-1241. The entire disclosure of this report is incorporated herein by reference.
A significant solution to this problem is found in my commonly assigned, co-pending U.S. patent application Ser. No. 07/925,193, now U.S. Pat. No. 5,262,880, the disclosure of which is hereby incorporated herein by reference. Here a sharp cut-off IR filter is employed. This filter, while cutting off the infrared region of the spectrum, does not cut off a portion of the visible red light. The resulting display thus can pass the NVIS-B criteria of Military Standard MIL-L-85762A, incorporated herein by reference.
U.S. Pat. No. 5,143,433, illustrated by FIGS. 15, discloses a backlit liquid crystal display panel which is readable via the unaided eye under bright lighting conditions of daylight and is also readable with night vision equipment (e.g. NVG) under darkened conditions of night. Fluorescent lighting tubes are used to provide high intensity light sources for daylight naked eye viewing of the display, and secondary low intensity light sources are used to allow for nighttime viewing of the display via night vision equipment.
Prior art FIG. 1 illustrates a preferred embodiment of the liquid crystal display panel backlighting system of U.S. Pat. No. 5,143,433, which utilizes primary high intensity fluorescent light tubes 18 either formed in a continuous serpentine fashion to attempt to achieve light distribution evenly over the liquid crystal display panel 12, or alternatively by using fluorescent light tubes 18 positioned parallel to one another behind the liquid crystal panel 12 of the display. A reflector body 24 is placed behind high intensity fluorescent tubes 18 to reflect light emitted in the reverse direction of the fluorescent tubes back towards the liquid crystal display panel 12 in a manner to encourage or enhance an even distribution of light intensity on the liquid crystal display. As shown in FIGS. 1-2, a reflecting surface 20 of reflector body 24 is sculptured to form cylindrically circular or parabolic reflection behind fluorescent tubes 18 as desired for the particular circumstance.
As shown in FIG. 2, light emitted during daylight operating conditions is shown by rays 30 emanating from the high intensity fluorescent light tubes 18. As shown, it is seen that forward or upward emitted rays proceed directly towards and through liquid crystal display panel 12 while the backside rays are reflected by surface 20 thereby being redirected toward the display panel 12. This illustrates the prior art normal manner of operation for daytime viewing where the liquid crystal display can be read with brightness and contrast. A diffuser plate 16 is placed in the path of the backlighting rays 30 before they reach the liquid crystal display panel 12. The diffuser 16 tends to smooth out the light intensity to aid in obtaining even intensity and wide viewing angles across the entire surface area of the LCD.
The embodiment of U.S. Pat. No. 5,143,433, as shown in prior art FIGS. 1-5, inserts within the reflector block low level light sources. For the particular embodiment shown in FIGS. 1 and 2, these low level light sources (22 and 38) are located along the axis of and behind each fluorescent light tube 18. By this arrangement, as shown in FIG. 2, low level light rays for use at nighttime emitted by light sources 38, represented by rays 32, are intercepted by the fluorescent light tubes 18 and reradiated for the most part as rays 34. This effect serves to provide a diffusing nature to the low intensity light reaching the liquid crystal display panel 12. The net result is a low intensity, diffused light impinging on panel 12. The further use of a diffuser 16 is not deemed necessary, although depending on the circumstances it is capable of being applied as required. The low level light sources 38 may be either incandescent, fluorescent lamps, or light emitting diodes.
FIG. 4 illustrates a second embodiment of U.S. Pat. No. 5,143,433 including a solid state optical plate 41 used as a light waveguide for backlighting the liquid crystal display. Fluorescent high intensity daytime light tubes 43 illuminate the edges of light guide 41. Light waves 45 emanating from fluorescent tubes 43 move through waveguide 41 by reflecting from one surface to the other back and forth across the width of the waveguide. The lower surface 47 of waveguide 41 is roughened slightly to cause a diffused scattering of light rays 45. This results in a diffused spread of light rays 49 upward in the direction of the liquid crystal display. This diffused reflection occurs continuously throughout wave guide 41 along lower surface 47. By this means, the liquid crystal display of this embodiment of U.S. Pat. No. 5,143,433 is illuminated from behind with a field of light across its surface area.
In FIG. 5, light guide 41 is shown configured on two sides by daytime high intensity fluorescent light tubes 43 and augmented on all four sides by distributed arrays 51 of low level intensity nighttime light sources. During daylight viewing, fluorescent light tubes 43 emit high intensity light rays through waveguide 41 to allow bright viewing of the LCD. For nighttime viewing through night vision equipment, high intensity fluorescent tubes 43 are turned off and low intensity light sources located in arrays 51 are activated. The installation of the low level light sources in arrays 51 is augmented with small night vision filters such as filters 53 shown in FIG. 3. As shown in FIG. 5, each array 51 houses a plurality of low intensity light sources and corresponding infrared filters such as those illustrated in FIG. 3.
Due to the structural arrangement of the backlighting system of U.S. Pat. No. 5,143,433, a large number of low intensity light sources and corresponding IR filters are needed. It would be advantageous to reduce the number of low intensity light sources and corresponding IR filters needed to properly illuminate a liquid crystal display during darkened conditions when NVG compatibility is required.
The liquid crystal display of U.S. Pat. No. 5,143,433 illustrated in FIGS. 1 and 2, also has an increased thickness or profile due to the positioning of nighttime low intensity light sources 38 and corresponding infrared filters 36 below the high intensity light tubes 18. In certain circumstances, such as cockpit liquid crystal display mounting, this added thickness is a disadvantage due to the spacial restrictions for instrument mounting.
The liquid crystal display of U.S. Pat. No. 5,143,433, as illustrated in FIGS. 4 and 5, has increased length and width dimensions due to the positioning of low level light sources 51 exterior to primary high intensity light sources 43. The low intensity nighttime light sources of FIGS. 4 and 5 cannot be edge-mounted except for the provision of light guide 41. Furthermore, due to the location of waveguide 41, the high intensity light sources 43 must also be edge-mounted adjacent the waveguide 41. Although providing an adequate day and night viewable liquid crystal display, the display of U.S. Pat. No. 5,143,433 takes up valuable space due to its large dimensions resulting from the addition and positions of the low intensity light sources. Furthermore, the LCD of FIGS. 1-5 utilizes a high number and redundancy of low intensity lamps, and a relatively high amount of total power is consumed by its lighting scheme.
Furthermore, the diffusers illustrated in FIGS. 15, while useful for improving the uniformity of projected light, deleteriously affect the intensity of the projected light resulting in the light appearing soft or washed out. Thus, additional high intensity lamps are required due to the employment of the light diffusers, resulting in an increased heating effect and power consumption upon the display.
For these and for other reasons there exists a need in the art for a liquid crystal display viewable both at night by night vision equipment (e.g. NVG) and during the daytime via the naked eye, which consumes a minimal amount of power, has a low profile, uses a minimal number of lamps, and a minimal number of infrared filters.
Furthermore, LCD backlighting systems with night lamps of lowered intensities are useful in commercial avionic application where low intensity dimming at night is advantageous for night convenience, absent NVG.
The term "low profile" is used herein in accordance with its well-known meaning in the art. Generally speaking, this term refers to a liquid crystal display which, through its thinness, does not take up inordinate space, often a critical characteristic or requirement to be met in avionics and aerospace vehicles. The term "low profile" may be defined by the term "LCD thickness" and/or "LCD size". "LCD thickness" is herein defined as overall display thickness including the matrix array, optics, backlight, ballast, and circuitry (e.g. when the elements of FIGS. 1, 6, and 11 are assembled together in an outside box, not shown). To be a "low profile" LCD, the LCD thickness should be less than about two inches, while the backlighting assembly thickness (e.g. elements 61, 63, 65, 72, 74, and 78 of FIG. 11) is preferably about 1.5 inches or less. The term "LCD size" is herein defined by the term "LCD thickness" plus the length and width dimensions of the overall LCD assembly.
The terms "substantially all infrared light" and "substantially all red light" are used herein together to mean that the infrared filter employed is one which creates a reasonably sharp cutoff between the near IR and red spectrum. An example of a filter with an unacceptable cutoff is reported in Abileah et al., "A Full Color AMLCD With NVG Class B Compatibility" IEEE AES Magazine (March, 1992), pp. 1237-1241, in FIG. 2, p. 1238. The result, as shown in FIG. 1, curve (2) of that article, is an unbalanced white color and a shift of the red color toward orange. An example of a filter with an acceptable, reasonably sharp cutoff is shown in FIG. 3, p. 1239 of that article. Such a filter, which only achieves a truly sharp cutoff for incident light at angles normal to its surface, may be obtained from WAMCO Corp (California, U.S.A.) as a "Wamco Night Vision Filter" and has the Spectral Table given in the aforesaid mentioned U.S. patent application Ser. No. 07/925,193, now U.S. Pat. No. 5,262,880.
It is a purpose of this invention to fulfill the above described needs, as well as other needs apparent to the skilled artisan from the following detailed description of this invention.